Eric Gourgoulhon wrote:
> - Technically, it would be desirable to create a subclass,
> ConstScalarField say, of the Element class of C^oo(U) (ScalarField) to
> implement constant fields, in order to take advantage of their
> specific properities
[...]
> - Mathematically, the set of constant scalar fields is a subalgebra of
> C^oo(U), so it would be natural to implement it as such. Is there any
> way to do this in the Parent/Element scheme ?
[...]
> In particular, CU(0).parent() would no longer be CU but CCU, which may
> result in some undesired effects...

How about defining two parents (CU and CCU, with a coercion CCU --> CU 
and a partial conversion CU --> CCU), and two element classes (SF and a 
subclass CSF), and having CU(0) create an instance of CSF, but set the 
instance's parent to CU?
 
-- 
Marc

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